Conventionally, various schemes of digital watermark technology have been developed as methods of protecting the copyrights of digital contents. These methods have recently received a great deal of attention as a technology for security and copyright protection in electronic distribution, in which pieces of handling information of digital contents, including the copyright holder name and the buyer ID, are invisibly embedded in the digital image information, thereby enabling to track use without permission by illicit copy. A digital watermark technology as a means for suppressing alteration of digital contents has also been developed. In this digital watermark technology, various data embedding methods have been proposed. In a method, information is embedded using a mask pattern. In this method, information is repeatedly embedded in digital image data in accordance with a mask pattern. For example, information is embedded at positions a, b, c, and d of each of mask patterns shown in FIGS. 1A to 1D using quantization error in accordance with mask pattern arrays shown as in FIGS. 2 to 4, thereby obtaining a synthetic image.
However, to improve the accuracy for specifying an altered portion in the resultant synthetic image, the mask patterns must be arrayed densely on the image data, as shown in FIG. 2 or 4. In addition, to improve watermark information detection accuracy in a partial image extracted from the synthetic image, generally, the mask pattern array as shown in FIG. 4 is preferably used. Hence, to simultaneously improve both altered portion specifying accuracy and watermark resilience against extraction, embedding is done using the mask pattern array as shown in FIG. 4.
This embedding method can improve the altered portion specifying accuracy and watermark resilience against extraction. However, this method suffers the following problems.
1) To improve the resilience, the mask pattern size is preferably as small as possible. However, low-frequency noise or block noise becomes noticeable to degrade the image quality. In addition, the embeddable information amount is limited by the number of data in the mask pattern.
For example, when the mask patterns shown in FIGS. 1A to 1D are used, the information amount is limited to four bits for the positions a, b, c, and d or 16 bits at maximum.
2) To improve the image quality, the mask pattern size is preferably as large as possible, though the resilience becomes poor.
3) To improve the altered portion specifying accuracy, the number of embedding positions must be large. However, low-frequency noise or block noise becomes noticeable to degrade the image quality.
That is, the image quality and the altered portion specifying accuracy/watermark resilience against extraction have tradeoff relationships. If one is improved, the other degrades: both cannot be simultaneously improved.
A digital watermark information embedding method called a patchwork method is known. In this method, the values of one part of an image are intentionally increased while the values of the other part are intentionally decreased. Hence, certain additional information can be embedded while the values of the entire image are kept almost unchanged.
Although it is conventionally known that information must be embedded undetectably for the human eye, the method of determining the image embedding position in the above patchwork method or the like has not been established yet.
To embed digital watermark information by partially modulating an image, for example, a method of determining the modulation position at random is available. However, with this method, the image quality cannot be kept sufficiently high.